1. Technical Field of the Invention
The present invention relates to a rectifier for an automotive alternator that is for use in a motor vehicle, such as a passenger car or a truck.
2. Description of the Related Art
A rectifier for an automotive alternator is generally designed to rectify three-phase AC power output from a three-phase stator winding of the alternator into DC power.
To be competitive in the market, it is required for the rectifier to be compact and light, have a high output and a low price, and be durable.
One of the factors that determine the durability of the rectifier is the fatigue life of solders that join, in each of the rectifying elements of the rectifier, a semiconductor chip to electrodes. Further, the fatigue life of the solders is influenced by loads, such as a load due to a difference in thermal expansion, a mechanical load induced during the assembly and installation of the rectifier. In particular, when a tensile load is applied to the solders in a direction perpendicular to the layers of the solder layers, the fatigue life of the solders will be considerably shortened.
To reduce such a tensile load, Japanese Patent First Publication No. 2004-282938 discloses a rectifier in which: a lead electrode of each of the rectifying elements is configured to include a stress-relieving portion; and a heat sink, which is disposed in contact with a mount electrode of each of the rectifying elements, is configured to have high rigidity.
In the above rectifier, when a large tensile load is applied to each of the rectifying elements, the stress-relieving portion of the lead electrode will be deformed, thereby absorbing the tensile load. However, when a small tensile load is applied to each of the rectifying elements, the stress-relieving portion of the lead electrode will not be deformed, allowing the tensile load to act on the solders. Consequently, as the small tensile load is continuously applied, the fatigue life of the solders will be shortened.
In addition, in the above rectifier, the stress-relieving portion of the lead electrode is formed into a spiral shape. However, forming such a complicated shape will increase the manufacturing cost of the rectifier.
Moreover, the lead electrode of each of the rectifying elements is generally soldered, at a high temperature, to a metal terminal so as to form a bridge circuit of the rectifier. However, when the temperature of the lead electrode drops from the high temperature to an ambient temperature, the lead electrode, which has joined in its expanded state to the metal terminal, will contract, producing a tensile load on the solders formed in the rectifying element. In particular, during operation of the rectifier, the lead electrode will be subject to a temperature lower than the high temperature for the soldering process, permitting the tensile load to constantly act on the solders.